This invention relates to magnetic tape transport, and particularly to such transports having provision for automatic threading of cassette-mounted tapes.
Magnetic tape transports adapted to automatic threading of cassette-mounted tapes are known in the art, e.g. U.S. Pat. Nos. 3,784,761, 3,831,198, 3,864,742, 3,866,856, 3,911,491, 3,940,791, 3,979,772, 4,035,842, 4,056,834, 4,074,329, 4,166,283, 4, 191,979, 3,984,870, and 4,101,944. In particular U.S. Pat. No. 3,940,791 (same assignee as the present application) shows automatic threading by means of differential air pressure apparatus, which pulls a loop of tape from the cassette and carries the loop 180 degrees around a rotating scanning drum having a pair of diametrically opposite scanning heads. The differential air pressure means keeps the shanks of the loop separated during the threading process, and a curved guide is then inserted in a direction axially parallel to the head drum to keep the radially outer shank away from the inner one, which is in contact with the scanning drum, before the differential air pressure means is shut off in preparation for recording or playing the tape.
U.S. Pat. Nos. 3,984,870 and 4,101,944 also show guides that are inserted into the loop as it is pulled around the drum by a mechanical threading apparatus.
Now, in the field of use for which the present invention is intended, it is desired to record a magnetic tape with, say, a television spot commercial of a few minutes length, the recording having been made on a high-quality studio recorder of the so-called "omega-wrap" type, and mount the tape in a cassette, then place the cassette in a magazine drum along with numerous others, for automatic selected replay in a television broadcasting studio. A machine for such use is disclosed in U.S. Pat. No. 3,720,794 wherein the rotating head drum has four heads and scans the tape transversely. To adapt such a machine for threading tape from a cassette and around an omega-wrap head drum (having only one head and a wrap of substantially 360 degrees) embodies grave difficulties. Of course, the transverse-scan rotating head cannot be arranged to follow the substantially diagonal track made by the omega-wrap head of the original recording machine. On the other hand, to provide an omega-wrap head structure in a machine of the complexity of that shown in U.S. Pat. No. 3,720,794 risks the production of low-quality results. The greatest problem lies in providing accurate positioning for the moving guides that operate to snug the tape close to the helical scan guide at the two closely adjacent ends of the substantially 344-degree tape wrap, i.e. at the "neck" of the omega where the tape sharply bends going to and from the drum. The guides must be so close that their movements tend to interfere. Such guides would have to move pivotally as well as linearly in order to properly position the tape around the scanner. The quality of the guiding, or lack of it, is extremely sensitive to the absolute position of these guides. The fact with known omega-wrap machines, all of which have fixed guides, a delicate adjustment is usually also provided to ensure their correct positioning. With movable guides, the problem of giving them accurate tape-guiding positions and inclinations at the end of their movement is substantially insoluble; or at least it is not soluble with the simplicity embodied in the present invention.
An example of the complexity involved in the automatic threading of an "alpha-wrap" machine (in which the tape makes a more-than-360-degree wrap) is shown in U.S. Pat. No. 4,191,979, and the critical problem of accurate positioning of the guides at the end of their movement is discussed in Column 12. The problem is much more severe in omega-wrap machines, since the end positions of the guides are much closer.
Accordingly, the present invention avoids the omega-wrap problem by using a two-headed 180-degree wrap in the playback machine, and this machine is especially arranged to be compatible with the one-headed 344-degree omega-wrap recording machine by making the 180-degree-wrap scanning drum twice the diameter of the omega-wrap drum, and by driving the heads at one-half the rotational speed of the omega-wrap head. Thus each of the 180-degree wrap heads scans the tape at the same head-to-tape speed, and covers the same length of track in the same time span, as does the single head of the omega-wrap machine. The simplicity of structure required for this novel arrangement and the fidelity of the result that can be produced, stands in sharp contrast with the complexity and low-quality result that must characterize the omega-wrap alternative.
It should be recognized that tape tension due to frictional build up will be different at various identical parts of the wording due to the differences in wrap angle between the present invention and the omega machine, that the effect of these differences can easily be overcome by use of the well-known "wide-window" time base corrector now extant.
It will of course be understood that the original recorder of the omega-wrap type is not subject to the same difficulties, since it is not required to handle cassette-mounted tape automatically, and can be threaded by hand over permanently-fixed guides.
Accordingly, it is an object of the present invention to provide a non-omega-wrap magnetic tape cassette transport of the automatic self-threading type that is compatible with and can interchange tapes with an omega-wrap machine.